1. Field of the Invention
The present invention generally relates to methods and systems for determining drift in a position of a light beam with respect to a chuck. Certain embodiments relate to a method that includes determining drift in a position of a light beam with respect to a chuck on which a specimen is disposed during inspection.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Fabricating semiconductor devices such as logic and memory devices typically includes processing a specimen such as a semiconductor wafer using a number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that typically involves transferring a pattern to a resist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a semiconductor wafer and then separated into individual semiconductor devices.
Inspection processes are used at various times during a semiconductor manufacturing process to detect defects on wafers. Inspection has always been an important part of fabricating semiconductor devices such as integrated circuits. However, as the dimensions of semiconductor devices decrease, inspection becomes even more important to the successful manufacture of acceptable semiconductor devices. For instance, as the dimensions of semiconductor devices decrease, detection of defects of decreasing size has become necessary since even relatively small defects may cause unwanted aberrations in the semiconductor devices.
Determining the positions of defects on the wafer is also important to facilitate processes such as defect review, during which locations on the wafer at which defects are located are re-visited to generate additional information about the defects. Therefore, if the determined defect locations are inaccurate, the defects must be searched for during review, which will reduce the throughput of the review process. In addition, inaccurate defect locations may reduce the accuracy and usefulness of review if other defects happen to be located at the inaccurate locations thereby confusing the results of inspection and review. Obviously, as the size of defects decreases, the acceptable error in the defect location also decreases. For example, the difficulty of finding defects based on inaccurate defect locations increases as the defect size decreases. Inaccurate defect locations will obviously affect any process that is performed based on defect location information such as defect repair or removal, defect analysis, etc.
One method for increasing the accuracy of defect detection and defect position determination is to accurately calibrate the inspection system prior to inspection of a wafer. For instance, during calibration of an inspection system, the offset of the light beam in the x and y directions from the center of the chuck on which a specimen will be located during inspection may be measured. The measured offset can then be used to correct positional information acquired during the inspection. Therefore, some calibration processes do account for drift in the light beam with respect to the chuck. However, since the calibration processes are typically not performed frequently (e.g., since frequent calibration will reduce throughput), any drift in the position of the light beam between calibrations is not measured. Instead, the incidence light beam position with respect to the chuck is assumed to be relatively stable between calibrations. Therefore, any drift in the position of the light beam between calibrations will produce error in the reported x and y coordinates of defect locations on the wafer.
Some inspection systems that use oblique incidence light beams are configured to account for some variation in the position of the light beam with respect to a wafer. The variation in the position of the light beam with respect to the wafer can be measured during inspection. Such variation is important to measure since variations in height can cause the position of the oblique incidence light beam to change on the wafer. Therefore, systems that can account for variation in the position of the oblique incidence beam due to height variation have provided an important correction for at least part of the drift in the oblique incidence beam position. However, accounting for drift in the x-y positions of the oblique incidence beam as described above only accounts for part of the lateral drift of the beam since a height change produces the same signal as a lateral beam drift requiring twice the radial position correction. As a result, half of the error introduced by drift in the position of the oblique incidence beam is not corrected.
Obviously, the position of a normal incidence light beam will not vary due to height changes. However, the position of the normal incidence beam may vary depending on, for example, drift in the optics of the system and drift in the chuck on which the specimen is located during inspection. The systems described above, however, do not account for any drift in the position of the normal incidence beam between calibrations. Therefore, although a normal incidence beam position will not suffer from positional inaccuracies due to the height variations described above, significant errors may be included in the positional information generated during inspection using a normal incidence beam. Such drift in the optics of the system and the chuck will also affect the positional information generated during inspection with an oblique incidence beam, and such inaccuracies will not be accounted for by the systems described above.
Accordingly, it may be advantageous to develop methods and systems for determining drift in a position of a light beam in the x and/or y directions with respect to a chuck on which a specimen will be disposed during inspection that can account for all possible variations in the position of the light beam with respect to the chuck and that can be performed relatively frequently without substantially reducing throughput of the inspection.